Moving bridge coordinate measuring machines, such as illustrated in FIG. 1 (prior art), include a base 110 having a planar base surface 111, and a moving bridge assembly 120 comprising two legs 121, 122 and a cross member 123 extending in what is usually labeled as the X axis and which spans over the planar surface of the base and connects the two legs, a cross slide 130 supported by the cross member and which translates across the cross member along the X axis, and a quill 140 carried by the cross member and which is translatable towards and away from the planar base surface in what is usually labeled as the Z axis. The moving bridge assembly travels across the base in what is usually labeled as the Y axis. A system of bearings (not shown) is typically located at the base of each leg of the moving bridge assembly. At least one of the legs 121, called the primary leg, typically has relatively wide width W and/or long length L (not shown) for the cross section along the height of the leg, and a base which accommodates a plurality of bearings located near the base of the primary leg to constrain pitch and yaw as well as vertical and horizontal straightness of motion. The other leg 122 of the moving bridge assembly, called the secondary leg, typically may have a relatively narrower width and/or shorter length for its cross section along the height of the leg, and a relatively simpler bearing assembly may be typically located near the base of the secondary leg, to constrain roll.
The relatively wide and long cross sectional design of the primary leg, while beneficial for providing stability, can restrict X axis motion of the quill as the cross slide and quill are translated across the cross member toward an end of the cross member connected to the primary leg. At the primary leg end of the moving bridge assembly, a part 150 being measured thus must be shifted away from the primary leg 121 to prevent interference between the part 150 and the quill 140. To prevent interference at the secondary leg end of the moving bridge assembly, the part 150 cannot extend into the volume swept by the secondary leg 122 as the moving bridge assembly travels in the Y direction. This has the effect of reducing the useable X axis measuring range and maximum part width in the X axis direction.
U.S. Pat. No. 4,682,418 describes an extension of the Y axis “bridge guide means” in conjunction with a table extension beyond the table to allow the probe (called a quill in the present application) to access the complete planar surface. These additional elements are all with respect to the fixed base member of the structure. The table extension adds cost and significantly increases the footprint of the measurement machine.
U.S. Pat. No. 4,908,950 describes an elongated slot (labeled 12 in FIG. 1 thereof) in the “front support arm” to allow mounting styli at that end of the bridge. There is no provision, however, for the provision of clearance for the probe (or quill) itself. Additionally, this elongated slot is not in the primary leg of the bridge.
U.S. Pat. No. 4,466,195 describes a bridge type measuring machine wherein legs 4, 9 of the moving bridge assembly have a relatively long base, but constant width along the height of the legs.
It would be desirable to provide a moving bridge type coordinate measuring machine which permits measurement of a part whose width covers the full area of the planar surface of the base of the machine without necessarily adding significant cost or increasing the footprint of the measurement system.